The invention concerns a clockwork stepping motor stator and a process for making same. The stator is of the type comprising a stator coil and a stator sheet extending through the coil. The stator sheet includes legs integrally joined adjacent their front ends by means of a pole shoe bridge arrangement. The rear ends of the legs are interconnected by means of a yoke sheet which is fastened to the legs.
A clockwork stator of this general type is known from DE-AS 25 59 635. A disadvantage of this stator is that only moderate efficiency of the clockwork stepping motor equipped with such a stator is attained, while a considerable manufacturing effort is required. These disadvantages originate at least to some extent from the fact that an axially short coil bobbin, which consequently must be of large diameter to hold the required coil amount, is molded directly onto a bridge or yoke part of the magnetic circuit. This does not result in optimum magnetic efficiency, since the outer layers of the winding of the stator bobbin contribute substantially less to the magnetization of the core than the inner layers. Furthermore, the use of high speed automatic coil winding machines is made difficult in the case of such a coil bobbin which is already equipped with a stator part. Another particular disadvantage may be that this bridge or yoke part of a multiple part stator sheet may have its magnetic properties affected adversely during the molding of the bobbin as the result of temperature and pressure effects. From a manufacturing standpoint, a disadvantage is that in the case of the known stator, a stator with widely open legs is connected in the area of the pole shoe only by very thin-walled webs bridging-over the rotor opening. These webs are easily broken or at least deformed, whereby the optimum geometry of the stator sheet in the pole shoe is now lost. Also, the mechanical deformation of this thin-walled web area of the magnetic circuit leads to a deterioration of the magnetic properties of the stator sheet exactly in the critical area of the rotor opening; for example, when the free ends of the widely opening U-geometry, after having been distorted in the course of the preceding processing (e.g., heat treating) steps, must be again aligned with the pairs of holes to be screwed to the ends of the bridge or yoke part. Finally, a considerable assembly effort is required in that a supplemental sheet forming an auxiliary pole must be placed in a high predefined position in the area of the rotor opening on the pole shoes of the stator sheet, in order to assure the necessary rotor starting torque in the rotor rotating direction desired.
Fundamentally, the same disadvantages are displayed by the prior art stator known from DE-OS 25 09 883, of a similar generic type and intended for stepping motors in small watches, wherein the starting conditions of the rotor are to be provided by the differently bordered sheets in the pole shoe area of a stator sheet stack.
From DE-AS 25 30 410 a basically similar stator is known, which displays the advantage of using stator coil of a greater length in relation to its diameter because it employs long stator sheet legs essentially parallel to each other. The pole shoes are formed on the frontal ends of the legs, so that a preferred magnetic direction due to a possible grain orientation of the preliminary material may be taken into account and utilized. However, a disadvantage involves the opening geometry in the area of the rotor opening between the pole shoes, which makes it difficult to obtain unambiguous geometrical conditions and the assurance of a geometrically optimum pole shoe range. This difficulty is enhanced by the fact that in order to mount the stator coil on the stator only a very thin stator sheet metal can be considered, because the legs of the U must be bent far apart; return of the legs then to their original geometry is difficult. Furthermore, this known stator requires an additional stator sheet (of different geometry in the pole shoe area) to insure the starting torque and the starting direction of the rotor; this condition, aside from the afore-mentioned problem of returning the bent structure to its initial geometry, requires great care in the mutual locating of the two different profile stator sheets.
In view of these conditions, it is an object of the invention to provide a novel stator and a process for the production of a stator of the above-described type, which may be manufactured simply and manipulated without criticality with respect to its parameters affecting its electromechanical efficiency, and which yields, while using a reduced amount of material, a highly efficient stator with a high reproducibility of its properties, in spite of the mass production of this precision mechanical product.
The object is attained by the present invention which relates to a stator and a process for making same. The stator is of the type comprising a stator coil and a stator sheet extending therethrough. The stator sheet includes legs which are integrally joined adjacent their front ends by means of a pole shoe arrangement. The rear ends of the legs are interconnected by means of a yoke sheet fastened thereto. The process comprises stamping out a plurality of stator parts in the form of a pair of integrally connected parallel legs which are each longer in length than width. The legs define an intermediate space therebetween. The stamp-out further includes a yoke sheet integrally connected to rear ends of the legs opposite the front ends. The legs and yoke are in a generally rectangular arrangement. The yoke sheet is separated from the rear ends of the legs, preferably after an initial treatment of the stamp-out (e.g., a heat treatment step), and thereby rendering the rear end of one of the legs accessible to the reception of the coil bobbin. The coil bobbin is inserted onto that one leg from the rear end thereof without moving the leg relative to one another. The separated yoke sheet is then secured to the legs adjacent their rear ends.
As noted above, another aspect of the present invention relates to the stator itself. The stator comprises a one-piece U-shaped stator sheet comprising a pair of parallel legs which are each longer in length than in width and which define an intermediate space therebetween. The legs form pole shoe bridges at front ends thereof for the reception of a driven rotor. The legs are integrally interconnected adjacent their front ends by one of the pole shoe bridges which extends across the axis of the intermediate space. A coil-carrying bobbin is mounted on one of the legs, with the axis of that bobbin being colinear with such leg so that a portion of the bobbin extends within the intermediate space. The length of the wound coil on the bobbin is longer than its diameter. A separate yoke sheet is fastened to the legs adjacent rear ends thereof to secure the bobbin on the leg and to form a return path for magnetic flux. The yoke sheet is positioned upon surfaces of the legs such that the plane of the yoke sheet is parallel to a plane containing the legs.
A separate aspect of the invention relates to the stator sheet itself which is adapted to receive the coil bobbin.